In general, liquid crystal display devices are mounted in personal computers, wordprocessors, pachinko machines, vehicle navigation systems, small-size TV sets, and the like, and have recently been in increasing demand. However, liquid crystal display devices are expensive, and hence demand for cost reduction has increased year by year. Of the components of a liquid crystal display device, a color filter exhibits a high cost ratio, and the demand for a reduction in the cost of the color filter has increased.
A color filter used in a liquid crystal display device is formed by arraying filter elements colored in, for example, red (R), green (G), and blue (B) on a transparent substrate. A black matrix (BM) for blocking light is provided around each filter element to improve the display contrast of the liquid crystal display device. BMs range from a BM using a Cr metal thin film to a recent resin BM using a black resin.
An overcoat layer (protective layer) made of an acrylic-based resin or epoxy-based resin and having a thickness of 0.5 to 2 μm is formed on a colored layer including a filter element to, for example, improve smoothness. A transparent electrode (ITO) film is further formed on this overcoat layer.
Various conventional methods of coloring the filter elements of a color filter are known, including, for example, a dyeing method, pigment dispersion method, electrodeposition method, and printing method.
In the dyeing method, a water-soluble polymer material as a dyeing material is formed on a glass substrate and patterned into a predetermined shape by photolithography. The obtained pattern is dipped in a dyeing solution. This process is repeated for R, G, and B to obtain color filters.
In the pigment dispersion method, a pigment-dispersed photosensitive resin layer is formed on a transparent substrate by a spin coater or the like. The resultant layer is then patterned. This process is performed once for each of R, G, and B, i.e., repeated a total of three times for R, G, and B, thereby obtaining R, G, and B color filters.
In the electrodeposition method, a transparent electrode is patterned on a substrate, and the resultant structure is dipped in an electrodeposition coating fluid containing a pigment, resin, electrolyte, and the like to be colored. This process is repeated for R, G, and B to form color filters.
In the printing method, a thermosetting resin in which a pigment-based coloring material is dispersed is colored by offset printing. This process is repeated for R, G, and B to form color filters.
The above color filter manufacturing methods have a common feature that the same process must be repeated three times to color layers in three colors, i.e., R, G, and B, and hence the cost is high. In addition, since a large number of processes are required, the yield decreases.
In order to eliminate these drawbacks, color filter manufacturing methods using an ink-jet system are disclosed in Japanese Patent Laid-Open Nos. 59-75205, 63-235901, and 1-217320. The ink-jet system is a method of forming filter elements by injecting coloring materials containing R, G, B color materials onto a transparent substrate using an ink-jet head and drying/fixing the coloring materials. In this method, since R, G, and B portions can be formed at once, simplification of the manufacturing process and a reduction in cost can be achieved. In addition, since the number of steps is smaller than those in the dyeing method, pigment dispersion method, electrodeposition method, printing method, and the like, an increase in yield can be achieved.
In a color filter used in a general liquid crystal display device, black matrix opening portions (i.e., pixels) for partitioning the respective pixels are rectangular, whereas ink droplets discharged from an ink-jet head are almost circular. It is therefore difficult to discharge ink in an amount required for one pixel at once and uniformly spread the ink in the entire opening portion of the black matrix. For this reason, a plurality of ink droplets are discharged to one pixel on a substrate to color it while the ink-jet head is scanned relative to the substrate.
As variations in the amounts of ink filled in the respective pixels are small, a high-quality color filter with reduced unevenness can be manufactured.
The amount of ink discharged from an ink-jet head may vary among nozzles even in discharge driving operation under the same discharge driving condition owing to variations in the structures of nozzles constituting the head or structures associated with discharging operation, driving mechanisms, and driving characteristics. In this case, even if the same numbers of ink droplets are discharged to the respective pixels, the amounts of ink filled in the respective pixels vary because of the use of different nozzles. The variations in the amounts of ink filled lead to unevenness among the pixels, resulting in reductions in the quality and yield of color filters.
In order to solve this problem of density unevenness, the following two methods (bit correction and shading correction) have been adopted. Consider here an ink-jet head for discharging ink using heat energy.
A method (to be referred to as bit correction hereinafter) of correcting the differences in ink discharge amount between the respective nozzles of an ink-jet head IJH which has a plurality of ink discharge nozzles shown in FIGS. 11 to 13 as disclosed in Japanese Patent Laid-Open No. 9-281324 will be described first.
First of all, as shown in FIG. 11, ink is discharged from, for example, three nozzles, i.e., nozzle 1, nozzle 2, and nozzle 3, of the ink-jet head IJH onto a predetermined substrate P, and the sizes of ink dots formed on the substrate P by the ink discharged from the respective nozzles are measured, thereby measuring the amounts of ink discharged from the respective nozzles. In this case, the width of a heat pulse applied to the heater of each nozzle is kept constant, and the width of a pre-heat pulse is changed. With this operation, a curve like the one shown in FIG. 12 can be obtained, which represents the relationship between the pre-heat pulse width and the ink discharge amount. Assume that all the amounts of ink discharged from the respective nozzles are to be unified to 20 ng. In this case, it is obvious from the curve shown in FIG. 12 that the width of a pre-heat pulse applied to nozzle 1 is 1.0 μs; to nozzle 2, 0.5 μs; and to nozzle 3, 0.75 μs. By applying pre-heat pulses with these widths to the heaters of the respective nozzles, all the amounts of ink discharged from the respective nozzles can be unified to 20 ng, as shown in FIG. 13. Correcting the amounts of ink discharged from the respective nozzles in this manner will be referred to as bit correction.
FIGS. 14 and 15 are views showing a method (to be referred to as shading correction hereinafter) of correcting density unevenness in the scanning direction of the ink-jet head by adjusting the ink discharge density from each ink discharge nozzle. Assume that as shown in FIG. 14, when the amount of ink discharged from nozzle 3 of the ink-jet head is set as a reference, the amount of ink discharged from nozzle 1 is −10%, and that from nozzle 2 is +20%. In this case, while the ink-jet head IJH is scanned, as shown in FIG. 15, a heat pulse is applied to the heater of nozzle 1 once for nine reference clocks, a heat pulse is applied to the heater of nozzle 2 once for 12 reference clocks, and a heat pulse is applied to nozzle 3 once for 10 reference clocks. With this operation, the number of ink droplets discharged in the scanning direction is changed for each nozzle, and the ink densities in the pixels of the color filter can be made constant in the scanning direction, as shown in FIG. 14. This makes it possible to prevent density unevenness of each pixel. Correcting ink discharge density in the scanning direction in this manner will be referred to as shading correction.
As methods of reducing density unevenness, the above two methods are known. For example, in a conventional color filter colored in the respective colors in a stripe pattern like the one disclosed in Japanese Patent Laid-Open No. 8-179110, the shading method, which is the latter of the above two methods, is used to adjust the discharge pitch on a pixel array basis so as to adjust the discharge amount for one pixel array. In this striped color filter, a color mixing prevention wall is provided between color pixel arrays to prevent ink of a predetermined color discharged to one pixel array from flowing into an adjacent pixel array of a different color.
In a color filter in which no color mixing prevention wall is provided between color pixel arrays and only a BM (black matrix) is provided as a partition between pixels, unlike a color filter as described above which is colored in a stripe pattern with a color mixing prevention wall being provided between color pixel arrays, when ink is discharged in the form of a line on a pixel array basis, the ink discharged onto the water-repellent BM flows into an adjacent pixel area, resulting in difficulty in managing the amount of ink discharged into each pixel.
That is, it is difficult to control the amount of ink applied into a pixel to a predetermined amount by using a method of adjusting discharge intervals as in the above shading correction.
With an increase in the resolution of color filter pixels, the pixel area tends to decrease. This makes it more difficult to control the amount of ink filled in each pixel.
For this reason, it is important to take new measures to improve the quality of a color filter in association with density unevenness by using the method (bit correction) of making discharge amounts uniform, which is the former method of the above two density unevenness reducing methods.
More specifically, in the form of adjusting ink filling amounts on a pixel basis instead of a pixel array basis, since it is expected that the amounts of ink filled in the respective pixels can be effectively made uniform by the above bit correction, it is required to realize uniformization of ink filling amounts by the bit correction using the simplest arrangement.
The first challenge to manufacture such a high-quality color filter is how to make the amounts of liquid filled in predetermined areas (pixels) uniform by bit correction.
The amount of ink discharged from one nozzle is influenced by whether or not ink is discharged from an adjacent nozzle at the same timing; the discharge amount changes depending on whether or not ink is discharged from the adjacent nozzle at the same timing. In this specification, this phenomenon will be referred to as nozzle crosstalk. In order to make ink discharge amounts uniform and eliminate unevenness between pixels, consideration is preferably given to discharge variations due to this adjacent nozzle crosstalk.
FIG. 35 shows a measurement result on adjacent nozzle crosstalk, by which the present invention is motivated.
FIG. 35 shows how the discharge amounts of a plurality of nozzles (80 ch in this case) of the ink-jet head vary when control is performed to advance and retard the discharge timing or make nozzles discharge or not discharge ink. FIG. 35 shows, in particular, the influence of the above adjacent nozzle crosstalk on discharge amount variations. More specifically, referring to FIG. 35, the discharge amount of the Nth nozzle (ch12), of all the nozzles (80 ch), is taken into consideration, and the discharge amount of this nozzle of interest is measured. In this discharge amount measurement, a voltage for driving the nozzle of interest (ch12), its current, and its pulse waveform are kept constant in all measuring operations. FIG. 35 shows a measurement result obtained when the discharge timings of neighboring nozzles are changed with respect to the discharge timing of the nozzle of interest (ch12).
Referring to FIG. 35, reference symbol (a) denotes the discharge amount of the ch12 nozzle obtained when ink is simultaneously discharged from all the nozzles (80 ch). This discharge amount is assumed to be 100 and is plotted as a right bar graph.
Reference symbol (b) denotes the discharge amount of the ch12 nozzle obtained when ink is discharged from selected half (40 ch) of all the nozzles (80 ch). In this nozzle selection, ink is simultaneously discharged from the ch11 and ch13 nozzles adjacent to the ch12 nozzle. In this case, the discharge amount is smaller than the discharge amount (a) by 1%.
Reference symbol (c) denotes the discharge amount of the ch12 nozzle obtained when ink is discharged from 40 ch nozzles, of the 80 ch nozzles, which are different from those selected in the case of “(b)”. In this nozzle selection, no ink is discharged from the ch11 and ch13 nozzles which are adjacent to the ch12 nozzle. In this case, the discharge amount is smaller than the discharge amount (a) by 5%.
Reference symbol (d) denotes the discharge amount of the ch12 nozzle obtained when ink is discharged from the same nozzles as those selected in the case of “(c)” of the 80 ch nozzles. In this nozzle selection, no ink is discharged from the ch11 and ch13 nozzles which are adjacent to the ch12 nozzle. In addition, ink is discharged from the remaining nozzles (39 ch) other than the nozzle of interest (ch12) with a delay of 10 μsec relative to the nozzle of interest (ch12). In this case, the discharge amount is smaller than the discharge amount (a) by 7% and smaller than the discharge amount (c) by 2%.
Reference symbol (e) denotes the discharge amount of the ch12 nozzle obtained when ink is discharged from only the ch12 nozzle of the 80 ch nozzles. In this nozzle selection, the discharge amount becomes smaller than the discharge amount (a) by 12%. Conversely, when ink is simultaneously discharged from all the 80 ch nozzles, the discharge amount of the ch12 nozzle is larger by 12% than that when ink is discharged from the ch12 nozzle alone.
Reference symbol (f) denotes the discharge amount of the ch12 nozzle obtained when ink is discharged from selected 40 ch nozzles, of the 80 ch nozzles, which are different from those in the case of “(d)”. In this nozzle selection, ink is discharged from the ch11 and ch13 nozzles adjacent to the ch12 nozzle. In this case, ink is discharged from the remaining nozzles (39 ch) other than the nozzle of interest (ch12) with a delay of 10 μsec relative to the nozzle of interest (ch12). In this case, the discharge amount is smaller than the discharge amount (e) by 7%.
In the case of “(g)”, although ink is discharged from all the nozzles (80 ch), ink is discharged from all the nozzles other than the nozzle of interest (ch12), i.e., the remaining nozzles (79 ch), with a delay of 10 μsec relative to the nozzle of interest (ch12). In this case, the discharge amount is smaller than the discharge amount (e) by 9%.
The cause of the above phenomenon can be explained as inter-nozzle crosstalk due to the propagation of the pressure wave of ink from an ink chamber 114 to each liquid channel 110. That is, as compared with the case of “(e)” wherein ink is discharged from the nozzle of interest alone, in the case of “(a)” wherein ink is simultaneously discharged from the 80 ch nozzles, pressure waves of discharged ink from the remaining nozzles (79 ch) other than the nozzle of interest (ch12) enhance discharging of ink from the nozzle of interest (ch12), resulting in an increase in discharge amount in the case of “(a)”.
In the cases of “(b)” and “(c)”, since ink is simultaneously discharged from 40 ch nozzles, an increase in discharge amount is smaller than that in the case wherein ink is simultaneously discharged from 80 ch nozzles. As compared with the case of “(c)”, in the case of “(b)”, since ink is discharged from the adjacent nozzles, the discharge amount increases to the same extent as this difference between these two cases. That is, whether or not ink is simultaneously discharged from adjacent nozzles has the greatest influence on discharging of ink from the nozzle of interest (ch12).
When the cases of “(a)”, “(e)”, and “(g)” are compared, it is found that the discharge amount of the nozzle of interest (ch12) changes as the discharge timing of a nozzle other than the nozzle of interest (ch12) is changed. As compared with the case of “(e)”, when ink is discharged from the remaining nozzles simultaneously with the nozzle of interest as in the case of “(a)”, the discharge amount increases. In contrast to this, as compared with the case of “(e)”, when ink is discharged from the remaining nozzles at a timing slightly retarded from the discharge timing of the ch12 nozzle as in the case of “(g)”, the discharge amount of the nozzle of interest decreases. This is because the interference phase of pressure waves produced by the remaining nozzles is reversed and acts to cancel out the discharge pressure produced by the nozzle of interest.
Likewise, when the cases of “(b)”, “(e)”, and “(f)” are compared, it is found that the discharge amount of the nozzle of interest changes as the discharge timing of the remaining nozzles other than the nozzle of interest is changed.
In addition, when the cases of “(b)”, “(e)”, and “(f)” are compared, variations in the discharge amount of the nozzle of interest (ch12) with respect to the differences in discharge timing among the remaining nozzles are smaller than those when the cases of “(a)”, “(e)”, and “(g)” are compared, to the extent by which the number of remaining nozzles other than the nozzle of interest (ch12) is smaller.
In addition, variations in the discharge amount of the nozzle of interest (ch12) with respect to the differences in discharge timing among the nozzles other than the nozzle of interest are influenced most by the nozzle adjacent to the nozzle of interest. When the cases of “(c)” and “(d)” are compared, nozzles separated from the nozzle of interest by three of more nozzles have some influence on the variations in discharge amount.
As described above, discharging/non-discharging of ink from nozzles other than the nozzle of interest and the discharge timing of these nozzles influence the amount of ink discharged from the nozzle of interest. However, no consideration has been given to these influences. When the number of nozzles to be used, the combination of nozzles to be used, or the discharge timing of each nozzle changes, the discharge amount of each nozzle changes. Such discharge amount variations may cause density unevenness among pixels. When, therefore, a high-quality color filter is to be manufactured, it is preferable that consideration be given to discharge amount variations due to the above adjacent nozzle crosstalk.
In addition, even if the discharge amounts of the respective nozzles are made uniform by bit correction before a pattern is formed or printed, discharge amount variations may occur due to the above adjacent nozzle crosstalk. It is therefore preferable that consideration be given to this point.
As described above, the second challenge to manufacture a color filter with higher quality is how to make the amounts of liquid filled in predetermined areas (pixels) uniform in consideration of discharge amount variations due to adjacent nozzle crosstalk.
In the above description, a color filter has been exemplified as an object to be manufactured. However, the first and second challenges arise not only in the manufacture of color filters but also in a case wherein the amount of liquid applied to a predetermined area (pixel) on a substrate must be controlled to a predetermined amount. For example, similar challenges arise in a case wherein a predetermined amount of EL (electroluminescence) material liquid is applied from a liquid discharge head (ink-jet head) to a predetermined area on a substrate to manufacture an EL display device. In addition, similar challenges arise in a case wherein a predetermined amount of conductive thin film material liquid (liquid containing a metal element) is applied to a predetermined area on a substrate to manufacture an electron-emitting device obtained by forming a conductive thin film on a substrate or a display panel including a plurality of such devices.